1. Field of the Invention
The present invention relates to an electronic ignition apparatus suitable for use in an internal combustion engine to prevent erroneous ignition due to ignition noise superimposed on ignition signals.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a typical conventional ignition system 11. This system 11 mainly includes an ignition signal generator 1 for generating an ignition signal in accordance with the rotation of an internal combustion engine (not shown); an ignition circuit 2 for producing, in response to the received ignition signal, an ignition pulse to energize an ignition coil 4 only for an adequate time period on the basis of driving conditions of the internal combustion engine; and a power transistor 3 triggered by the ignition pulse to energize the ignition coil 4. The above-described ignition circuit 2 and the power transistor 3 belong to an igniter circuit 10. In general, the igniter circuit 10 is fabricated practically by the use of a thick-film integrated circuit. The ignition signal 1 is incorporated in a distributor.
FIG. 2 shows operating waveforms of signals in the individual circuits of the above-mentioned ignition system 11, in which: FIG. 2A is an ignition signal generated from the ignition signal generator 1; FIG. 2B is an ignition pulse outputted from the ignition circuit 2; FIG. 2C is a coil current flowing through the ignition coil 4; and FIG. 2D is a high ignition voltage derived from the ignition coil 4.
The ignition system 11 performs the following operation. First, the ignition signal generator 1 is driven synchronously with the rotation of the internal combustion engine and generates an ignition signal in proportion to the rotation rate of the engine. The ignition signal (FIG. 2A) is fed to the ignition circuit 2, which then produces ignition pulses (FIG. 2B) under so-called "duty ratio control" in conformity with the driving conditions of the internal combustion engine. The ignition pulse has a "high" level in each of durations t1-t2, t3-t4 and t5-t6. The NPN power transistor 3 is switched on and off repeatedly in response to these ignition pulses so as to be turned on at the "high" level of each ignition pulse, and also to be turned off at the "low" level thereof. Then, coil current shown in FIG. 2C flows in the ignition coil 4 by the switching operation of the power transistor 3, and an ignition voltage represented in FIG. 2D is applied to a spark plug (not shown) at the coil current interruption. Thus, the high ignition voltage is outputted at each of time instants t2, t4 and t6.
In the ignition system 11 with the above circuit arrangement, there may arise a problem of ignition noise if the ignition signal generator 1 and the igniter circuit 10 are not positioned in the distributor integrally as in a specific case where the igniter circuit 10 is mounted to a vehicle body separately from the distributor.
This ignition noise problem will now be described below with reference to FIG. 3. There are shown a harness 15 for connecting the primary winding 4A of the ignition coil 4 to the collector of the power transistor 3 in the igniter circuit 10 via a first signal terminal 10A of the igniter circuit 10, and harnesses 16 and 17 for connecting the ignition signal generator 1 to the ignition circuit 2. FIG. 3 is a typical conventional circuit corresponding to the ignition circuit 2 of FIG. 1. The circuit arrangement of this conventional ignition circuit 2 will be described in detail later.
As mentioned above, the harnesses 15 through 17 have a function of interconnecting the ignition signal generator 1, the ignition coil 4 and the igniter circuit 10, and practically some connectors (not shown in detail) are employed therein with the harnesses 15-17 bundled in wiring. As a result, the harnesses 15-17 are physically adjacent to one another to cause electrical interference. For achieving exact ignition at the spark plug, therefore, a shielded wire is used particularly for each of the harnesses 16 and 17 connected between the ignition signal generator 1 and the igniter circuit 10 to suppress electromagnetic induction of ignition noise from the harness 15 which is a primary signal line of the ignition coil 4. Similarly, a shielded wire is used also for the harness 15 to directly suppress ignition noise.
Practically, however, there still exist some non-shielded portions due to the connectors (not shown in detail) provided in the harnesses 15-17. Moreover it is actually impossible to shield the bodies of the connectors and the wire ends of the harnesses inserted therein. Consequently, in a structure where the igniter circuit 10 is incorporated separately from the ignition signal generator 1, resultant ignition noises are superimposed on the ignition signals fed to the igniter circuit 10. The longer the non-shielded portions of the harnesses 15-17 become, the more the ignition noises increase. The shielding effect is dependent also on the joint performance of the shielded wire used in each connector.
Now a further detailed description will be given below on the ignition noise caused by the ignition and erroneous ignition resulting therefrom. In the internal circuit arrangement of the ignition circuit 2 shown in FIG. 3, there are included a voltage comparator 21, a first reference power source 22 for applying a first reference voltage to one input terminal 21A of the voltage comparator 21, a second reference power source 23 connected to a reference voltage terminal 1A of the ignition signal generator 1 and serving to apply a second reference voltage thereto, and a diode 24 connected between another input terminal 21B of the voltage comparator 21 and the ground. The diode 24 is so connected that a forward current flows therein when the input voltage for the other input terminal 21B of the voltage comparator 21 becomes a negative voltage. The ignition signal output terminal 1B of the ignition signal generator 1 and the other input terminal 21B of the voltage comparator 21 are connected to each other. The second reference voltage applied from the second reference power source 23 is fed to the voltage comparator 21 via a signal path established from the aforementioned reference terminal 2A of ignition circuit 2, harness 17, reference voltage terminal 1A of ignition signal generator 1, ignition signal generator 1, output terminal 1B of ignition signal generator 1, input terminal 2B of ignition circuit 2, and the other input terminal 21B of voltage comparator 21. Then the second reference voltage is compared with the first reference voltage obtained from the first reference power source 22, so that a basic, or low voltage ignition pulse is produced. The basic ignition pulse thus obtained is fed as a base signal to the power transistor 3 via a duty ratio controller (not shown in detail) which performs the duty ratio control in accordance with the driving conditions of the internal combustion engine.
FIG. 4 shows the detailed waveforms of signals obtained in the individual circuits of FIG. 3 at the time of generating the ignition voltage (FIG. 2D) by interruption of the coil current (FIG. 2C) flowing in the ignition coil 4. FIG. 4A is a base voltage of the power transistor 3; FIG. 4B is an operation mode of the power transistor 3; FIG. 4C is a collector voltage of the power transistor 3; and FIG. 4D is an input voltage to the ignition signal input terminal 2B of the ignition circuit 2. This waveform chart roughly represents a phenomenon on a scale of 10 microseconds. The reference voltage terminal 1A of the ignition signal generator 1 is supplied with the second reference voltage from the second reference power source 23, and the generated ignition signal (FIG. 2A) is superimposed on such second reference voltage. The ignition signal of FIG. 2A is fed to the other input terminal 21B of the voltage comparator 21 via the ignition signal input terminal 2B of the ignition circuit 2 and then is compared with the first reference voltage applied to one input terminal 21A, whereby the comparison signal is converted into the aforementioned basic ignition pulse. Subsequently, the ignition pulse is controlled by the duty ratio controller and then is inputted to the base of the power transistor 3 to switch the transistor 3, thereby energizing the ignition coil 4. More specifically, in a state of normal ignition, the base voltage of the power transistor 3 is inverted from "high" level to "low" level at an instant t10 in FIG. 4A, so that the power transistor 3 is switched off at the subsequent instant t11 as shown in FIG. 4B (the time period between t10 and t11 corresponds to a delay in the operation of the power transistor 3). Consequently, the collector voltage of the power transistor 3 is turned to "high" level during the time period of t11-t13 to produce a pulsed voltage. The ignition voltage generated at the secondary winding 4B of the ignition coil 4 is induced by such pulsed voltage after an instant t13 also at the primary winding 4A of the ignition coil 4 serving as a transformer, and the collector voltage thus induced appears with a half sine wave as shown in FIG. 4C. The phenomenon that the induced voltage of such waveform based on the ignition voltage appears at the primary winding 4A of the ignition coil 4, occurs when a discharge gap (not shown) of the spark plug connected to the secondary winding 4B of the ignition coil 4 has an infinite length. Assuming now that the harnesses 15 and 16 are positioned with a sufficient space kept therebetween, the input voltage to the ignition signal input terminal 2B of the ignition circuit 2 is entirely free from interference induction of the collector voltage of the power transistor 3 as represented by a solid line in FIG. 4D, so that the result waveform is not adversely influenced with induction of even the collector voltage pulse generated during the time period of t11-t13 during which the voltage variation rate is high.
To the contrary, in case the harnesses 15 and 16 are positioned to be directly adjacent to each other, a collector voltage pulse (t11-t13) of the power transistor 3 appears at the ignition signal input terminal 2B of the ignition circuit 2 due to induction from one harness 15 to another harness 16. Since such collector voltage pulse has a high voltage variation rate, it causes a greater inductive action than the collector voltage waveform succeeding to the instant t13, hence causing harmful ignition noise in the ignition circuit 2 (as represented by a broken line in FIG. 4D). The ignition noise increases due to induction as the physical space between the harnesses 15 and 16 becomes shorter or the mutual proximity thereof is kept over a longer distance. In addition to the above, such ignition noise is dependent also on the joint performance of each harness within the connectors.
When the ignition noise thus induced exceeds the operating voltage "Vr" of the voltage comparator 21 at the time of generation of the ignition voltage, the voltage comparator 21 produces an extra pulse (represented by a broken line in FIG. 4A) at the instant t12. As a result, such extra pulse unnecessarily turns on again the power transistor 3 which has been once turned off at the instant t11 (as represented by a broken line in FIG. 4B), whereby the ignition coil 4 is unnecessarily energized. The time period between t11 and t12 corresponds to the response delay of the circuits other than the power transistor 3. Since the collector voltage pulse (FIG. 4C) generated in the time period t11-t13 has a short duration of 10 microseconds or so, the ignition noise (represented by a broken line in FIG. 4D) appearing at the ignition signal input terminal 2B of the ignition circuit 2 is also as short as 10 microseconds or so. Therefore the extra on-time of the power transistor 3 starting with the instant t12 comes to be about 10 microseconds, so that the former off-state is resumed at an instant t15 after the lapse of several 10 microseconds from the instant t12. However, even at the instant t15 (corresponding to the normal off-instant of the transistor 3), a pulsed voltage similar to the one at the instant t11 (normal instant) appears in the collector voltage of the transistor 3, whereby the phenomenon of such pulsed voltage generation is successively repeated during the time period t11-t13. Since the repetition characteristic is different depending on the degree of electrical interference between the harnesses 15 and 16 as well as on the characteristics of the ignition system 11, the repetition cycle and frequency are not fixed. The erroneous ignition caused from the interference between the harnesses 15 and 16 is likely to occur more readily with regard to the harness 16 on one side than to the harness 17 on the other side, because the latter 17 is more proximate to the second reference power source 23 and has a lower impedance as compared with that of the harness 16. The operating voltage Vr is basically determined by the difference between the first and second reference voltages of the first and second reference power sources 22 and 23. Since the operating voltage Vr is controlled by the source voltages and the ignition signal period, there arises a problem of ignition noise. Accordingly, the operating voltage Vr is so set as to become high at the time of generation of the ignition voltage. However, in the electronic ignition circuit of FIG. 3 where the operating voltage Vr is at most several volts, there exists a drawback that, depending on the joint performance of the harnesses, the ignition noise reaches almost double the value shown in FIG. 4D and consequently satisfactory antinoise characteristic is not attainable.
In order to solve the above problem, there is conceived another conventional ignition system shown in FIG. 5. In this circuit, a resistor 25 is connected between the positive terminal of a second reference power source 23 and the reference voltage terminal 2A of an ignition circuit 2, a feedback circuit 31 containing a series connection of a resistor 32 and a capacitor 33 is connected between the reference voltage terminal 2A of the ignition circuit 2 and a pulse generator 36. The pulse generator 36 is provided for producing ignition pulses in response to the comparison signal of the voltage comparator 21 so as to optimally drive the power transistor 3. Precisely speaking, the pulse generator 36 outputs a first ignition pulse to drive the power transistor 3 and a second ignition pulse having an in-phase relation thereto to the above feedback circuit 31. Such individual generation of the ignition pulses as mentioned is based on the fact that the first ignition pulse for driving the power transistor 3 is influenced by the operation of the transistor 3 and its waveform and voltage are thereby varied. Since the feedback needs to be executed without any harmful influence from such variations, the first ignition pulse for driving the transistor 3 is not applied to the feedback circuit 31.
Merely the above circuit configuration alone is the difference in comparison with the first-mentioned ignition system of FIG. 3, and the remaining circuits are exactly the same as those included in the system of FIG. 3. Therefore, the explanation is omitted here.
FIG. 6 shows the operating waveforms of signals obtained in the individual circuits of FIG. 5, in which: FIG. 6A is a base voltage of the power transistor 3; FIG. 6B is its operation mode; FIG. 6C is its collector voltage; FIG. 6D is a feedback signal; and FIG. 6E is an input voltage to the ignition signal input terminal 2B of the ignition circuit 2. The base voltage of the power transistor 3 is inverted from "high" level to "low" level at an instant t20 and subsequently, is turned off from its on-state at an instant t21 after the lapse of a predetermined time period (t20-t21). Simultaneously with inversion of the base voltage of the power transistor 3 from "high" level to "low" level, the pulse circuit 36 produces its output by inverting the feedback signal (FIG. 6D) from "high" level to "low" level. The feedback circuit 31 produces a differential voltage V.sub.FB in response to the feedback signal from the circuit 36 and superimposes the feedbacked differential voltage V.sub.FB on the output of the second reference power source 23, whereby the second reference voltage obtained from the second reference power source 23 is rendered lower by a value corresponding to the differential voltage V.sub.FB. Consequently, the voltage at the ignition signal input terminal 2B of the ignition circuit 2 also becomes lower in accordance therewith since the low component of the differential voltage V.sub.FB serves as a negative component to the second reference voltage. Succeeding to the instant t20, the differential voltage V.sub.FB from the feedback circuit 31 gradually approximates to the high component in conformity with its differentiation characteristic. A pulsatory collector voltage of the power transistor 3 appears at the instant t21, which will become an ignition noise in the input voltage to the ignition signal input terminal 2B of the ignition circuit 2. The differential voltage V.sub.FB from the feedback circuit 31 has a sufficiently low value even during the time period between the instants t21 and t22 in which the ignition noise is generated. As a result, the peak value of the ignition noise never exceeds the operating voltage Vr of the voltage comparator 21, and therefore an erroneous inversion of the power transistor 3 to its on-state by the ignition noise is not caused unlike in the aforementioned first example of the prior art at the instant t12 (FIG. 4B).
Thus, in the second conventional ignition system of FIG. 5, the ignition noise can be substantially masked by the feedback voltage from the feedback circuit 31 of a simple configuration. However, the ignition pulse derived from the pulse circuit 36 is inputted to the feedback circuit 31 also at its rising portion (not shown in FIG. 6) from "low" level to "high" level and then is superimposed on the second reference voltage of the second reference power source 23. This phenomenon occurs during energization of the ignition coil 4 and exerts harmful influence on the energizing characteristic (i.e. duty ratio characteristic) of the ignition coil 4. Consequently, in the second conventional ignition system of FIG. 5, there still exists another drawback that execution of the feedback for merely preventing the ignition noise only is not sufficient.
In other words, according to the last-mentioned conventional ignition system illustrated in FIG. 5, another problem is raised, although it is practically possible to solve the aforementioned ignition noise problem that is induced by the high voltage ignition pulse voltage on the wiring of harnesses 15-17. Another disadvantage is such that the duty ratio characteristic of the ignition coil is deteriorated.
Now such deterioration of the duty ratio characteristic will be described below with reference to the graphic representation of FIG. 7, in which the duty ratio is plotted along the ordinate and the rotation along the abscissa.
In FIG. 7 showing the duty ratio characteristic of the ignition coil 4, a solid line represents basic characteristic (i.e., desired duty ratio characteristic obtained without the feedback circuit 31). In case the feedback circuit 31 is present, the amount of feedback increases with the duty ratio having a greater value as represented by a broken line, so that the ignition coil 4 is energized in surplus correspondingly and it brings about a surplus current consumption in the coil 4 or thermal break down thereof.
As is obvious from the above description, the second conventional ignition system shown in FIG. 5 has advantageous effects to prevent the erroneous ignition if the ignition noise superimposed on the ignition signal is relatively small. However, a great amount of feedback is required for maintaining a normal operation if any great ignition noise appears. In the latter case where unnecessary energization of the ignition coil is executed, extra current consumption and heat generation are caused in the ignition coil to eventually raise a problem of its thermal break down.
The present invention has been accomplished in an attempt to solve these conventional drawbacks, and an object to provide an improved electronic ignition apparatus which is adapted for use in an internal combustion engine and is capable of completely preventing erroneous ignition against any great ignition noise while performing proper energization of an ignition coil in conformity with desired duty ratio characteristic.